Rival militias clashed in Tripoli leaving at least seven people dead, Libyan media said, as AFP journalists reported hearing gunshots and explosions in the south of the capital.
Tanks, trucks and heavy weapons were seen on the streets but the cause of the conflict was not immediately clear.
It was not immediately possible to confirm the death toll.
Tripoli is controlled by a loose alliance of militias of different political and religious factions, and clashes between them are an almost daily occurrence.
Five years after Libya's 2011 revolution that toppled longtime dictator Moamer Kadhafi, the country is embroiled in violence and political chaos as the militias and two rival governments vie for power.
A UN-backed Government of National Accord took up office in March with the hopes of re-establishing central power and stemming a jihadist threat from the Islamic State group.
It was intended to replace the rival administrations but has failed to win a vote of confidence from the parliament in Tobruk while the Tripoli-based GNA considers it illegitimate.
The GNA suffered a fresh setback in October after the rival Government of National Salvation seized key offices in the capital.